There are prep products that remove it - the problem is in previous
paintwork if it's been trapped in layers, old dried out paint that has
soaked it up (yes, paint does dry out and become absorbent), scratches, and
so on. It hides and it is very difficult to ever get every bit of it. Add to
that the occasional worker that forgets to clean with prep BEFORE even
touching down with a piece of sandpaper, and it then gets into the paint
where prep no longer pulls it out. Evil, evil stuff. I remember my days in
doing restoration work on old English sports cars and Porsches, in the
middle of laying down the paint there would be a big fish-eye, which made me
scream bloody murder!